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 Trappist-1 as seen from one of its planets Trappist-1 as seen from one of its planetsAn exoplanet, or extrasolar planet, is a planet outside our Solar System that orbits another star. The first detection of an exoplanet was confirmed in 1995. Named 51 Pegasi b, it is a giant planet, not unlike Jupiter. Since then, more than 4000 exoplanets, belonging to more than 3000 different solar systems, have been discovered. In August 2016, an Earth-sized world was detected orbiting Proxima Centauri, the nearest star to the Sun. A number of stars have exoplanets orbiting in what is known as the “habitable zone”, a region lying at a certain distance from the star within which a planet’s surface temperature will allow liquid water—and therefore potentially life—to exist. Scientists estimate that there are as many as 40 billion potentially habitable Earth-sized planets in the Milky Way Galaxy. With so many of these worlds, the chances of life existing elsewhere in the Galaxy seem high.

An artist's impression of Kepler-62f, an exoplanetAn artist's impression of Kepler-62f, an exoplanet
HARPS at the ESO, La Silla, ChileHARPS at the ESO, La Silla, Chile

Detecting exoplanets

More than 2300 exoplanets have been discovered by the Kepler Space Telescope, while the High Accuracy Radial velocity Planet Searcher (HARPS), installed on the 3.6-metre telescope at the European Southern Observatory (ESO), La Silla, Chile, has discovered about a hundred.

Exoplanets are far too faint to be observed directly. The Sun, for example, is about a billion times as bright as the reflected light from any of the planets orbiting it. The glare from the parent star also makes it all but impossible to make out a faint light source lying close to it. Nearly all exoplanets have been detected through indirect methods. 

The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi.

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